Retrofit for Wound Rotor IM's Control 2

[powerjunx]

Obselete spare parts are main reason for every justification ever made (often) for replacement, revision and retrofitting of a certain electrical equipment.

Above is pertaining to a late '50's built WRIM and controls which uses Static Stepless rotor winding connected to saturable core reactor (i guess) where a resistor bank (connected to a secondary WRIM) is invariable while in series with secondary winding of the reactor. Operations are:
1.)Upon half-way pressed start button (unlatch), motor runs smoothly at low speed and goes with magnetic clutch brake when button is release.
2.) Further pressing the button as full way pressed (unlatch), motor changes smoothly to high speed as in Stepless speed control (for WRIM as cited). Magnetic clutch brake is also activated as 1.).

To note, the operation is forward and reverse. I hope somebody had come across with this.

A planned was initiated to replace the system with VFD and new SCIM. Anyone can help me for parameter setting preference? Though, i had one but i'm not confident enough
if it will work out to restore the sequential operation of old/existing system as above.

Here's my prefered setting:
1.) JOG/Inching Mode at first start at low speed while employing "DC Injection" when button is release.
2.) Further pressing the button will change the speed to high speed. Upon button is release, half way upward, speed will take back to low speed. A complete release of the button will halt the motor as initiated by DC injection.
Thus, i would prefer to use 2 preset speed (low,hi), DC injection(at 3sec), and JOG(at low speed). Will this work? Anyone could point out loopholes?

Thanks,
bill

 

[jraef]

Not a bad plan overall. 2 comments:
1) You don't necessarily need to replace the WRIM, you can short out the rotor when using a VFD. If you do that, add in a good output filter so that the PWM output does not damage the stator windings in your old motor. If you already suspect your motor has problems, then it would make sense to replace it now however. If you do, get a motor designed for inverter operation.

2) DC Injection braking on VFDs is of limited use because it forces all of the kinetic energy in the load to be absorbed by the motor, mostly in the rotor where it has trouble getting out. For this reason it is usually restricted to the final 10% of speed. You should consider a dynamic braking resistor package with that drive which will absorb most of the kinetic energy (as heat in the resistors) until almost the last second, then program the drive to automatically use DC injection only when the speed is below 10%. This will prevent unnecessary heating of the windings.

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[CJCPE]

Although DC braking is good for low speeds, zero torque is produced at zero speed. Note that this curve shows how torque varies with speed, but a VFD is not going to put out enough current to get the torque level shown.

 

[jraef]

Good point CJCPE. It does provide braking torque right UP TO zero speed, just not holding torque, and he didn't say what the application is.

So billybry, if your application was to use the braking to hold the load, such as in a winch or hoist (a common use for WRIMs) neither DC injection nor Dynamic braking will work for that. You will need mechanical brake. You may still want to do the majority of the braking with the VFD however, then just use the mechanical brake as a "parking brake", which will see virtually no wear and tear because most or all of the motion is removed by the VFD.

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[powerjunx]

Thanks, jraef likewise CJCPE.

Regarding to application, this motor is part of 50/10 ton Hoisting System package particularly as a BRIDGE MOTOR (Long travel motion)where a frequent start - stop sequence mostly occurring, though its intermittent.

How about synchronizing both clutch brake and DC injection?
Else, VFD has Fast stop feature will this work in liue of DC injection?

 

[CJCPE]

You can probably coordinate the clutch and brake with one or more drive features. You probably don’t need the clutch but the brake may be good to have. It is hard to match a friction brake for providing a dead stop and holding torque at zero speed. The standard VFD stopping sequence is to ramp down the speed on a linear ramp to the lowest controllable speed and then de-energize and let the motor coast. In many VFDs the de-energize/coast speed is available as a setup adjustment. You should also be able to get an output at that point to control the brake.

The fast stop feature is probably just a faster ramp setting that takes effect during the stop sequence but not for deceleration due to reducing the operating speed setting. You may be able to get several ramp rates that take effect at various speeds. Whenever the operating speed is reduced, the motor will inherently provide regenerative braking if the ramp is set faster than the natural coast-down rate. The motor will become an induction generator and return energy to the drive. A standard drive has no way of returning the energy to the AC line and the DC bus caps will charge to an excessive voltage unless the regeneration is limited to the small amount that gets dissipated as losses. Some drive’s simply trip off line if the decal rate is set too fast. The better solution is to automatically override decel setting. An override system that works well will allow a fast decel setting but actually stop the motor with the fastest reliable speed/time profile.

A dynamic braking circuit would switch in resistors to dissipate regenerative energy and increase the available braking torque.

You might want to calculate the inertia reflected to the motor shaft and figure out how much help you get from the friction of the drive train. You should be able to figure out quite a bit if you can run some tests on the current system releasing the clutch without applying the brake at various speeds and checking to see how far and/or how long it coasts.

 

[powerjunx]

CJCPE, thanks! A plan is suppose to use Telemecanique Altivar 28 series. Any input details on its features that somehow fits
to restore the existing application, cited above?
Jraef, there was a great deliberation over WRIM - SCIM replacement issues. We come out with SCIM inverter duty. Thanks!

 

[jraef]

Ah, you should have mentioned that it was a small motor at the outset, I would have recommended replacing it straight away. For something 20HP and under, the cost of messing with the WR system to convert it to SCIM would not be worth it.

The ATV28 is new for Schneider but it appears to be capable of your basic functions. I don't see that it is a vector drive however (it might be, but they don't mention it one way or the other), which may leave you unsatisfied with low speed stability. I am very familiar with the ATV71 and I think that if your budget can stand it, you would be in a position of having a VFD capable of whatever you need it to do. Just a thought.

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[powerjunx]

Thanks, jraef. You're right! It's 20 HP WRIM. what a good guess..
Upon our discussion, i heard that it's a sensorless vector drive (i rather google it in the net).
Regarding to my application, i think this would suffice (ATV) where it just conveying the load of the hoist to forward or reverse motion. There will be no initial load as in hoisting system where a mechanical brake releases after a while the motor develops a full torque to left the load. In these case, i would rather use this drive. Any comments?

 

[ddcemf]

If this is a hoisting application, I suggest you look at a crane-specific VFD. I can't speak for requirements outside the USA.

For a hoist application, two forms of braking are required. Typically what is used is a motor brake (like a Stearns disc) AND a Weston mechanical load brake. If your hoist does not have the Weston brake, your drive will definitley need to be a flux vector drive with encoder feedback, something that is capable of floating a load--to produce 100% holding torque at zero speed.

If you have the secondary brake, your options are more wide open.

Crane-specific VFD are always going to run a premium of general purpose drives on account of the custom control software flash.

 

[powerjunx]

ddcemf, with due respect, you're missing the point.
This is a part of "P&H" 50/10 ton Hoisting System package particularly as a BRIDGE MOTOR (Long travel motion).
i'm thankful for your comments.

regards